1. Field of the Invention
This invention relates to an improved touch panel membrane switch. More particularly, it relates to a transparent membrane switch having improved optical properties.
2. Prior Activities in the Field
Several flat panel switch mechanisms have been proposed. See, for example, the article "Touch screens diversify" by J. D. Logan in ELECTRONIC PRODUCTS, 28, 11 (Nov. 1, 1985) which describes capacitive, resistive membrane and LED or optical types of touch screen swtiches and the booklet MEMBRANE KEYBOARD DESIGN MANUAL distributed in 1983 by the Dorman Bogdonoff Corporation.
These switches share a number of characteristics and preferred applications. For one, they are basically two dimensional having a thickness of less than a millimeter and a length and width many times that size. They are usually multilayer and, in the case of capacitive and resistive membrane types, involve several electrically conductive layers. They all respond to finger touch. This response can take the form of deflecting one layer to make contact with another layer such as deflecting a top layer to contact a base so as to effect a physical engagement of electrical contacts or of deflecting a layer to bring about a measurable change in capacitance or resistance or the like.
It is often desirable to make the touch panel switches transparent so that they can be placed in front of an illumination source and back lit. In a very preferred application, the illumination source is a cathode ray tube and the switch is positioned directly in front of or adhered to the face of the cathode ray tube. This arrangement allows the user to activate the switch in direct response to information appearing through the switch from the cathode ray tube.
Representative patents in the area of touch panels and their use with cathode ray tube displays include U.S. Pat. Nos. 3,560,675; 3,673,327; 3,757,322; 4,110,749; 4,186,392; 4,220,815; 4,230,967; 4,305,071; 4,310,839; 4,346,376; 4,413,314; 4,423,299; 4,427,861; 4,449,029; 4,459,476; 4,484,179; 4,516,112; 4,517,559; 4,521,870; 4,542,375; 4,553,142 and 4,567,480.
These back-lit touch panel switches operate well in the absence of direct light. However, most settings where these switches are to be used do have direct light percent. In fact, in many settings, the touch panel itself carries fixed text which must be read with direct light in conjunction with the text passing through the touch panel. This can lead to problems. The layers commonly employed in touch panel switches are inherently reflective. In the case of the capacitive and resistive switches they include either a myriad of fine wires or they contain thin but conductive films of metal which can be very reflective. Thus, if adequate direct light is present to permit text printed on the touch panel to be readily read, there is often such a level of reflection that the information coming through the switch is not deciferable. This problem can be made worse if the touch panel is equipped with an electromagnetic shield, a radio frequency interference shield or an electrostatic discharge shield, all of which are becoming more prevalent in cathode ray tube device designs and all of which can contribute to undesired reflections.
A number of remedies to this problem have been proposed. One is to increase the brightness of the backlighting cathode ray tube. This is often difficult and costly. Another has been to apply an antireflective coating or layer on the outside of the panel switch. This is probably the current method of choice.
It is an object of this invention to provide an improved touch panel membrane switch which operates with improved efficiency in conditions of simultaneous back light and direct light.